In LTE (Long Term Evolution), handover control is performed in order for a mobile station UE in “RRC_Connected state” to perform communication constantly by using the most suitable cell (see Non-patent Document 1, for example).
In the handover control, when the mobile station UE performs measurement control to detect a cell having better quality than that of a current cell, the mobile station UE reports this event to a radio access network. The radio access network sends the mobile station UE “HO command” for transitioning to the detected cell. Then, the mobile station UE which received the “HO command” attempts to establish connection to the cell designated as a transition target. FIG. 4 shows a sequence of the above-described handover control.
Meanwhile, In LTE Release-10, CA (carrier aggregation) to perform communication by aggregating multiple CCs (component carriers) under the same radio base station eNB, namely, “Intra-eNB CA” has been introduced in order to realize broadband communication over 20 MHz (for example, communication at 100 MHz) (see FIG. 5(a)).
Thereafter, in LTE Release-12 and beyond, “Small Cell enhancement” has been proposed and introduction of “Inter-site CA (Inter-eNB CA)” to perform communication by aggregating CCs (cells) under different radio base stations eNB is under study as one type of architecture more flexible than the conventional one.
For example, one possible operation by using the “Inter-site CA” is to perform communication of a control signal (a C-plane signal) requiring reliability with a cell #1 (a macrocell) under a radio base station eNB #1 via an SRB (Signaling Radio Bearer), while to perform communication of a user data signal (a U-plane signal) requiring broadband communication with a cell #11 (a small cell) under a radio base station eNB #11 via a DRB (Data Radio Bearer) (see FIG. 5(b)).
FIG. 6(a) shows signal paths of a C-plane signal and a U-plane signal where a mobile station UE is not performing the CA, and FIG. 6(b) shows signal paths of a C-plane signal and a U-plane signal where the mobile station UE is performing the “Inter-site CA” (an operation mode shown in FIG. 5(b)).